As illustrated in allowed U.S. patent application Ser. No. 910,534 by Richard E. Crandall, et al, filed May 30, 1978 and assigned to the assignee hereof, there is disclosed a multiwire intrusion detection system in which a number of sensors are coupled in parallel across a multiwire cable which connects the sensors to a central control unit. The central control unit provides a remote indication of an alarm condition as well as provided all necessary signals for the sensors such as carrier, reset, latch, freeze and mode select signals.
The sensors themselves may be of the ultrasonic, infrared or microwave variety, with the particular type of sensor being chosen for the particular type space which is monitored. For instance in the monitoring of hallways, it may be desirable to utilize an infrared sensor, whereas in the monitoring of large areas an ultrasonic sensor may be desirable. The use of microwave sensors may be desirable in the case where the distance from sensor to the area to be monitored is greater than that which would be acceptable when utilizing an ultrasonic sensor.
Situations sometimes arise in which it is desired to provide volumetric protection, but the environment is such that no single sensor can be configured to perform at a low false alarm rate. For example, in an entry foyer of a department store, an ultrasonic detector can be disturbed by air currents leaking around the outside doors. Moreover, a microwave unit can be troubled by reflections and the penetration of the microwave signal through glass, causing sensitivity to vehicles on the street. Passive infrared detectors can be affected by direct or indirect sunlight.
As described in U.S. Pat. No. 4,195,286 issued to Aaron A. Galvin on Mar. 25, 1980, the use of redundant sensors is extremely effective in reducing the false alarm rate associated with a single sensor in a given location. In general what is described in this patent is a system in which two or more sensors monitor the same area. An alarm signal is sent only when an alarm condition is sensed at both sensors. The applications for such a redundant system are their use in very sensitive areas, for example, in nuclear fuel storage facilities, where agency response might have to be massive. Were not some redundancy utilized to combat false alarms, the false alarm rate would be intolerable. Redundant sensors can also be utilized to advantage in very severe environments where volumetric detection is required.
In summary, no matter how reliable a single sensor is, it will produce some measurable level of false alarm through a combination slight misapplication and occurrance of statistically infrequent but possible spurious events. Once the sensor is installed, it is only with extreme difficulty that any measurable reduction in false alarms can be achieved. When a reduction can be achieved, it is usually at a substantially reduced detection likelihood.
More particularly, by requiring the alarming of two or more sensors which have differing false alarm mechanisms, another dimension becomes available to reduce greatly the false alarm likelihood with a minimal sacrifice in intruder detection performance. It can be shown that assuming each detector operates with a probability of detection of 98%, e.g. it will fail to detect intrusion an average of 2% of the time, and assuming that each detector false alarms an average of once in three years and that its typical minimum time to alarm is one second, in which the target or spurious event must exist in the detection zone for at least one second in order to produce an alarm, then the false alarm rate for a redundant system can be shown to be once every 1,000 years.
However, one of the basic difficulties in structuring a system which utilizes redundant sensors at only a few locations compared to the locations covered by the entire system, is the problem of configuring a universal adapter which will permit the system to operate in its normal non-redundant mode without change, while at the same time providing that for those locations at which redundancy is required, redundancy can be achieved very simply with the utilization of an adapter into which the redundant sensors are plugged. Thus it is desirable to have an adapter which can be connected in normal fashion across the multiwire cable utilized in the system, and provide an alarm signal which duplicates the alarm signals normally generated by the single sensors, the existence of which indicates that an alarm condition has been detected at both of the two redundant sensors.
In the past, interface modules have been used to connect different types of sensors to a multiwire cable. One such system is described in U.S. patent application Ser. No. 910,534 filed by Richard E. Crandall et al on May 30, 1975 for a Multiple Sensor Intrusion Detection System and assigned to the Assignee hereof. Here an interface module is provided for each sensor so that different types of sensors can be accomodated. However this interface module does not accomodate redundant sensors.